Why people add the I before certain words?

Like I-Robot, iPhone, IFreeApps

Apple used that name so everyone tried to copy them.

- iAnonymous.

So they can ask more money for it.It's the I from I.T…

In the case of Apple products, the "i" originally stood for, "internet." Getting internet access on computers was much more difficult in the 90s than it is today, and the iMac was marketed as a computer which made internet connectivity easy. Apple then started applying it to their other products (which did admittedly make sense in the case of the iPhone). Apple products became highly successful, so, as Señor iAnonymous said, everybody tried to copy them.